1. Field of the Invention
The present invention relates generally to electric resistance vacuum heat treating furnaces, and more particularly to improvements in a high temperature electric resistance vacuum furnace suitable for heat treating processes, such as brazing, tempering, degassing, sintering and hardening, in which the hot zone is heated by radiant energy and cooled by recirculated fluid.
2. Description of the Prior Art
Electric vacuum heat treating furnaces typically consist of a cylindrical water-cooled vessel containing heating elements forming a hot zone for receiving, a workload to be heat treated. An example of such a furnace is disclosed in U.S. Pat. No. 3,438,618 to Seelandt in which a cylindrical vessel contains a retort of separate upper and lower water-cooled, U-shaped shells with end walls movable into side-by-side relationship to form a box-like chamber. Radiant heating elements line each shell in transverse planes axially spaced along the length of the chamber. Additional elements in flat grids line both end walls. Multiple nested layers of radiant heat-reflecting shields reflect some of the radiation from the elements back into a hot zone work space. The furnace is evacuated by an oil diffusion pump to provide a non-oxidizing atmosphere during the heat treating process. A quenching fluid of inert gas may be injected into the chamber after the heating phase of the process is completed and recirculated through a heat exchanger for rapid cooling. U.S. Pat. No. 4,559,631 to Moller teaches annular banks of heating elements in planes axially spaced in the furnace. The banks of elements may be differentially located and/or energized to establish front-to-rear temperature trim zones. U.S. Pat. No. 3,185,460 to Mescher et al. and U.S. Pat. No. 3,257,492 to Westeren disclose elongate heating elements coaxially mounted in the furnace and mutually spaced from each other.
The heating elements are usually fabricated in flat bars of graphite or refractory metals such as commercially pure molybdenum in rectangular cross section as shown in Moller, supra., Seelandt, supra, proposed another element design which is elliptical in cross-section and of substantial thickness. The convex surfaces of the element face inwardly toward the middle of the chamber and outwardly toward the heat shields.
While prior art electric vacuum furnaces as above-described are satisfactory for many heat treating processes, they are lacking in certain design features which significantly improve efficiency in the process. Heating elements of thin rectangular or elliptical cross sections are prone to sag under high temperatures between spaced apart supports because of low section modulus. The rectangular and elliptical elements also inherently lack even distribution of emitted radiant energy from all surfaces for achieving the precision demanded. The radiant energy is emitted in opposite directions substantially perpendicular to the flat sides, consequently, energy directed toward a heat shield is merely reflected back to the element instead of onto the workload. Elements with elliptical or similarly curved surfaces direct only a portion of the radiant energy emitted toward the heat shield for reflection onto the workload. The above-described heating element designs choke a significant percentage of the emitted radiant energy which reduces the effective surface area and results in higher element temperatures causing creep, sagging and non-uniform heating. Hence, the temperature of the workload will not be of optimal uniformity and a relatively long heat treating cycle time is required. When quenching fluid is recirculated in the furnace through a heat exchanger at completion of the heat treating phase, the extremely hot fluid returning to the heat exchanger may heat seals and other components therein beyond their design limits causing permanent damage and leakage.
Accordingly, it is a general object of the present invention to provide an electric resistance vacuum furnace suitable for heating a workload to high temperatures with better uniformity and for cooling the workload and furnace without damage to component parts of a recirculating cooling system.
Another object is to provide a high temperature vacuum furnace utilizing electric radiant energy heating elements of substantial stiffness with minimal cross sectional area that will not sag under high temperatures between horizontally spaced apart supports.
Still another object is to provide a furnace design for clean high vacuum operating conditions where heat is applied in a very uniform and controlled manner for heat treating processes such as brazing, tempering, degassing, sintering and hardening.
A further object is to provide an arrangement of heating elements which will efficiently disperse radiant energy from a high percentage of surfaces of the elements to a workload within the furnace.
Still another object is to provide an electric vacuum furnace wherein re-circulation of cooling fluid is regulated to prevent exposed temperature sensitive components from exceeding designed limits.
Still another object of the invention is to provide a furnace construction which meets the severe demands of the heat treating industry for precise temperature trim control during the heating phase of a process.
These and other objects, novel features, and advantages of the invention are accomplished in a high temperature vacuum furnace having a hot zone formed by longitudinally aligned matching parallel pairs of radiant energy heating units evenly spaced around the sides of the furnace starting with two adjacent pairs across the top, and opposed pairs continuing down the sides and two adjacent pairs across the bottom. Matching pairs of units at the front and back ends of the hot zone are arranged at multiple elevations. Each pair forms a trim zone which is automatically regulated both radially and longitudinally according to the temperature required by the workload in that zone. The units of each side pair comprise two parallel aligned resistance elements electrically connected in series at their one ends, and the units of each end pair comprise parallel aligned elements connected in series. Each element has lengthwise surfaces angularly disposed from each other to form a beam structure having a relatively high section modulus for stiffness and resistance to sagging. Also, the angles of the element surfaces facing a heat shield assembly effectively radiate a high percentage of the energy toward the assembly for reflection into the hot zone in addition to the direct radiation from the element surfaces facing the hot zone. The furnace includes a re-circulating cooling system for cooling of the furnace and workload in a controlled manner that reduces distortion of the workload. An inert gas cooling fluid bypasses the hot zone interior passing instead around the outside of the heat shield assembly and through a heat exchanger until the circulated fluid temperature drops below the maximum tolerated by all component parts in the cooling system, after which the fluid flow is modulated to pass directly through the hot zone interior.
The foregoing features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.